Magnetic recording media, typically those used for backing up computer data, require enhanced recording density to meet increasing storage capacity requirements. A magnetic powder of small particle volume is considered necessary for achieving high recording density. Assignee has responded to these circumstances by establishing the manufacturing method set out in Patent Document No. 1 shown below and developing various other improvements in metal magnetic powder production technology.
As can also be seen in the literature published heretofore, the main constituent of metal magnetic powders is typically iron. Industrial manufacture of iron-system metal magnetic powder is generally carried out by the method of incorporating a sintering inhibitor such as Si, Al, rare earth element or an alkaline earth metal element into an acicular powder comprised mainly of iron oxy-hydroxide or iron oxide and then reducing the powder.
Earlier attempts to enhance magnetic powder properties have focused mainly on how to improve the properties of the magnetic powder itself and on how to prevent sintering and thus improve dispersibility. Patent Documents Nos. 2 to 5 define the atomic ratio of rare earth element and the like on the particle surface and teach that magnetic recording media excellent in electromagnetic conversion characteristics can be obtained by using magnetic particles falling with the defined ranges. Of particular interest is Patent Document No. 5, which defines the amount of sintering inhibitor per unit surface area and teaches that inclusion of sintering inhibitor at or greater than the prescribed value is necessary to prevent particle adhesion and enhance dispersibility, and thereby improve the magnetic properties and surface properties of the magnetic recording medium.
In order to boost the recording density of a magnetic recording medium, it is necessary to increase the number of magnetic particles included per unit volume and therefore necessary to reduce particle size. Patent Document Nos. 7 and 8, for example, teach use of a magnetic powder whose particles are reduced to the finest possible. The particle-size refining method used is generally to carry out particle-size refinement at the stage of the starting material (precursor) for synthesizing magnetic particles by firing and reduction. However, when the refined precursor is subjected to firing and reduction, the likelihood of inter-particle sintering, axial ratio degradation owing to particle shape deterioration, and other such problems tends to increase. This has made it necessary to include a large amount of sintering inhibitor in the precursor.
However, the rare earth element, Al and Si used as sintering inhibitors are nonmagnetic, so that increasing the relative content of these “nonmagnetic constituents” per unit volume of the magnetic powder lowers the saturation magnetization. As explained above, the need to increase the amount of sintering inhibitor arises particularly when the particle size of the magnetic powder is refined, so that there has been a problem of the saturation magnetization being markedly degraded owing to increase in the amount of sintering inhibitor per unit volume. Further, Patent Document No. 9 teaches a method of increasing dispersibility by subjecting the magnetic particles to compression deaeration for decoupling the bonds caused by sintering and inter-particle action.
On the other hand, reduction of particle volume (size) by refinement generally lowers saturation magnetization. One reason that can be given for this is that the particle surface needs to be formed with an oxide layer of a certain thickness for maintaining the weatherability of the magnetic powder, so that the percentage of particle volume accounted for by the metal component decreases with higher particle-size refinement. An effective way to improve the magnetic properties of a metal magnetic powder is to increase the percentage of the volume accounted for by the metal portion, i.e., the magnetized portion, and this has been a common avenue of approach in the past. However, the method employed focuses on regulating the thickness of the oxide layer, and in this case a problem remains in that weatherability is degraded owing to the fact that the relative thickness of the oxide layer diminishes.    Patent Document No. 1: JPA-07-022224    Patent Document No. 2: JPA-06-215360    Patent Document No. 3: JPA-07-078331    Patent Document No. 4: JPA-07-184629    Patent Document No. 5: JPA-2003-296915    Patent Document No. 6: JPA-2005-101582    Patent Document No. 7: JPA-2003-242624    Patent Document No. 8: JPA-2005-259929    Patent Document No. 9: Japanese Patent No. 3043785